Controlling the valves of internal combustion engines by mechanical-hydraulic means has been previously proposed--see German Patent Disclosure Document No. DE-OS 29 26 327. A camshaft is used which controls the valves and, interposed in the transmission path between a cam follower in contact with the camshaft and the valve itself is a hydraulic cushion, or a fluid cushion or plug, which is provided by a hydraulic pressure source. It is possible to change the valve open and closing times to match the valves to specific operating conditions of the internal combustion (IC) engine by including a drain valve in the hydraulic system which includes the cushion so that, even though the mechanical cam may be in a position tending to hold the valve open, hydraulic transmission fluid has been drained or bypassed, so that the valve can return to closed position, for example under influence of a spring, in advance of the rotation of the cam to "valve-closed" position. The operation of the valve, thus, can be matched to conditions of the engine, for example to lower fuel consumption by dropping the choking losses arising in passage of gases around the valves, or to decrease valve timing overlap in various cylinders of a multi-cylinder engine at low speeds. Additionally, starting of IC engines can be improved.
The control system which has been proposed utilizes a rotary slider, driven from the camshaft of the engine, and so arranged that its angular position with respect to a predetermined angle or reference position of the camshaft can be changed within some limits. The rotary slider then controls the hydraulic system which can decrease the open-time commanded by the mechanically driven camshaft-cam follower arrangement, opening the valve. Thus, and upon decrease of the open-time, better operation of the engine can be obtained.
Electronic control of the timing of the open-time of the valve would be highly desirable; electronic control can utilize sensed signals which can be processed in accordance with operating characteristics of the engine. It has not been possible to utilize the advantages of electronic control without excessive requirements of apparatus and the like which transfer the processed electrical signals to output elements, such as servo positioning elements operating on the valves directly. The precision with which the valves can be controlled, for example in accordance with a desired position of the rotary slider previously described, will depend on the precision of the transfer system. This requires extremely close tolerances in manufacture, and even minor tolerance differences will introduce transfer errors. Such transmission errors are practically unavoidable in mass production apparatus, and calibration or adjustment to compensate for tolerances is difficult, time-consuming, and not always reliable. Independent, variable control of the valves of respective separate cylinders of a multi-cylinder IC engine, and possible disconnection of cylinders from the power train, cannot be carried out by the known system.